A few Apollo missions had a classified experiment onboard. I forget the name, and I have never been able to figure out what it was. It was certainly minor.

Chapel Bell? Over-the-horizon radar?

That's it. Was Chapel Bell an OTHR experiment?

The hints I've found were indicating it was a test of acoustic detection of rocket launchings, by existing and experimental DoD sensors around the world, using the launch as the 'signal'. As far as I can tell there was no hardware or any activity by the crew -- it was entirely passive.

Gemini-5 had a special DoD camera experiment that became one of Gordo Cooper's space tall tales. It used to be posted on www.space.combut that URL is now dead, so with permission i'm posting the whole item here:

Mercury astronaut Gordon Cooper, in his new book Leap of Faith, presents a tale of government cover-ups related to spy cameras, to Area 51, and to similar subjects top-secret subjects, based on his own personal experiences on a NASA space mission. As a certified "American hero," his credibility with the public is impeccable. But several space veterans who SPACE.com consulted about one of Cooper's spaceflight stories had very different versions of the original events. And some of them showed me hard evidence to back up their skepticism. According to Cooper, in 1965 he carried a super-secret spy camera aboard Gemini-5 and accidentally got some shots of Area 51 in Nevada. Consequently, the camera and its film were confiscated by the Pentagon, never to be seen again. He was personally ordered by President Johnson not to divulge the film's contents. ďOne special mounted camera we carried had a huge telephoto lens," he wrote. "We were asked to shoot three specific targets from our spacecraft's window because the photo experts wanted to be able to measure the resolution of the pictures. "That's exactly what we did: Over Cuba, we took pictures of an airfield. Over the Pacific Ocean, we took pictures of ships at sea. Over a big U.S. city, we took pictures of cars in parking lots. Beyond that, we were encouraged to shoot away at other airfields, cities, and anything else we wanted along the way." In an exclusive interview with SPACE.com, NASA's former chief photo analyst, Richard Underwood, confirmed the existence of the experiment but remembered details about it in a very different way than Cooper did. Now retired in Nassau Bay, Texas, Underwood recalled the camera was a 35-mm Questar with a Zeiss 'Contarex' lens. That is a "cataoptic system" (folded optics), with a foot-long barrel giving "several thousand" mm's of focal length (Cooper recalls it was 1250mm). Mounted on the spacecraft window, it was shot at 1/50th of a second at various ground targets passing directly below the spacecraft. "It was the same camera that Ed White took outside with him on his space walk in June," Underwood recalled. "We just slapped a big lens assembly on the front end." Considering the optical characteristics of the camera, Underwood calculated that the theoretical maximum resolution -- the smallest object discernable in photographs -- could be as small as a hundred feet or so. That was what the Pentagon was hoping for. Cooper, however, distinctly remembers differently about the image quality. "After splashdown," he wrote, "and while Pete and I were still aboard the recovery vessel, the exposed film from that mounted camera was rushed to a darkroom and developed. I was shown a few pictures -- including some unbelievable close-ups of car license plates." Absolutely impossible, retired NASA space photography experts have told SPACE.com. "We never unloaded or developed space film on recovery ships," Gene Edmonds told SPACE.com from his home in LaPorte, Texas. "We always kept it safe for development in our own specialized labs back in Houston." Nor, say space reconnaissance experts, would license plates be readable from a handheld camera in space -- or any other satellite in the 1960s. Writing recently in the Washington Post, Dr. Dwayne A. Day, a civilian specialist in space reconnaissance, had criticized Hollywood movies for exactly this kind of exaggeration. "The best resolution of an American spy satellite, achieved by an older series no longer in use, was reputed to be about 2 1/2 inches," Day wrote. "This means that the smallest visible object would be the size of a baseball, not the thin letters and numbers on a license plate." Day also told SPACE.com that this vehicle didn't show up until many years after Gemini-5. According to Day and other experts space.com talked with, Cooper's claim violates the laws of optics. "How could he even aim it?" one asked derisively. "You can't visually make out parking lots from orbit". Nevertheless, NASA's Dick Underwood does recall that the 1965 Pentagon experiment's film was indeed grabbed while the crew was still on the recovery carrier. "Cooper was really upset," he added with a grin, confirming Cooper's account. "I was livid", Cooper had written, "but there was nothing I could do." But Underwood and his associates remember a lot more about what became of the film than Cooper does. That is not surprising since it was their specialization to handle space photographs. As far as Cooper ever knew, the images had totally vanished, and that's the way he's been telling the story for years. Not so, retorts Underwood. NASA was sent a set of prints from the roll after the Pentagon developed it and studied it. "Nothing too exciting showed up," he recalled. Those pictures wound up filed right along with the other NASA-sponsored shots from Gemini-5, Underwood continued. "The NASA shots were all on 70-mm Hasselblads," he explained, "and the 35-mm shots in the archives came from the Pentagon experiment." As Underwood recalls, more than a decade later, when astronaut Tom Stafford returned to military duty as a brigadier general and was assigned to direct space planning at the Pentagon, as a favor to Underwood he inquired after the original negatives. They were nowhere to be found. "He was told they must have been destroyed", reported Underwood. Not at all, recalls another retired NASA photo expert. "They sent back the original negatives later when they realized they weren't very good," Tom Brahm told SPACE.com. "When I retired, they were still somewhere in the archives in Houston." And that's exactly where I found these photos when I visited the photographic archives in Building 424 at the Johnson Space Center. Archive director Mary Wilkerson had collected the documentation and transparencies for my inspection. Two rolls of 55 shots each, and 28 more from a third roll, had been catalogued in the NASA photo system on September 23, 1965, just a month after the flight, according to archivist David Sharron. The original documentation indicated the shots had never been classified, and from examining them it was easy to see why. Each shot covered an area several miles on a side, with sharpness a bit better than a typical television screen. That's equivalent to a ground resolution of perhaps 50-100 feet, exactly what Underwood had estimated. The views showed precisely what Cooper recalled observing: airfields, islands, even a large US city (Dallas, it turned out). The pictures of airfields showed runways, but aircraft and buildings couldn't be made out. On the city shot, it was impossible to see small letters, or even license plates, or even cars, or even parking lots at all. A crisscross pattern of roads and city blocks could be made out, but nothing smaller. The catalog for the NASA archives described one other problem. There was little if any crew documentation of where the photos were taken. "The astronaut log is sketchy and difficult to use for any data of value," it said. Dr. Day had also heard of this problem when he had researched the history of US spy satellites. "What happened with Gemini was that the astronauts were sloppy in recording where they took their shots," he explained. "The guys at the National Photographic Interpretation Center determined that any manned reconnaissance system would still require a computer to keep track of where the camera was pointing and when it was shooting. People naturally started to ask 'If we have a computer taking the photos anyway, why do we need the guys?'" For a number of similar reasons, the Pentagon lost interest in a manned military space station (the Manned Orbiting Laboratory, or MOL), and cancelled the program a few years later. The poor photography on Gemini-5 wasn't the main cause, Day continued, "but it was one of the early nails in the MOL coffin." Cooper, meanwhile, never learned the photographs had been returned to NASA by a disappointed Pentagon. He later told a story about how somebody had explained the confiscation to him. "Many years later," he wrote in his book, "at a 1997 NASA reunion at Cape Canaveral, a gray-haired man came up to me and asked if I remembered him." The man identified himself as the one who confiscated his film, and then asked, "Did anyone ever tell you why the film was confiscated?" Cooper wrote that the man told him, "You had the most magnificent pictures of Area 51". Cooper was thrilled: "As for Area 51, I hope the Air Force is conducting experiments with flights with highly unusual aircraft -- even saucers with revolutionary propulsion systems," he wrote. "And the first person to come home with pictures of the mysterious Area 51? An astronaut from space." Sadly, this may be yet another tale that doesn't stand up. It turns out, on Gemini-5 Cooper never flew over Area 51 at all. The spacecraft orbit always passed far to the south. It was physically impossible for him to have taken any near-vertical snapshots of the super-secret test range. Even a simple check of his trajectory shows this. The Gemini's orbital inclination was 33 degrees, and the southernmost regions of "Area 51" are at 37 degrees north. That's a horizontal distance of about 300 miles, from an altitude of about at most 150 miles. Anybody can use satellite tracking software widely available on the Internet, with specific Gemini-5 trajectory measurements (also on the Internet). They will find out that as viewed from Area 51, Gemini-5 never got higher than about 20 degrees on the southern horizon, barely above the mountains for only a few seconds, once or twice a day. As viewed from his spacecraft, that would be far out near Earth's horizon, lost in the dust and haze of the thick atmosphere. The camera experiment requirements, and Cooper's own memory of it, involved only straight-down views of the nearest portions of the surface, seen through the thinnest layer of the atmosphere. Area 51 would never have been imaged, even by accident, except by grossly violating the experimental procedures. There was one intriguing item in the catalog, however: roll B exposure 24 was called "California, north of Salton Sea," right before another shot of the Elephant Butte reservoir in New Mexico. That's in the direction of Area 51, at least. I pulled the transparency out of its envelope and loaded it in the viewer for close examination. But the picture, like most of the others, was another disappointment. Splotchy desert sands, covered by scattered clouds, showed only a few faint lines that might have been roads. There were no runways, no mystery buildings, no UFOs. Cooper's narrative -- the prematurely developed film with visible license plates, the forbidden peek at a super-secret installation, the cover-up from the highest levels of the government -- is by far the more exciting version of this incident. His sincerity and self-confidence remains unquestioned. His agent, Stu Miller, assured SPACE.com that the details of the story were true. "In fact, there were numerous 35mm cameras aboard Gemini-5, not just one as you postulated," he wrote (presumably the NASA camera experts forgot about the others, never listed them in the mission manifest or photo archives, and mislabeled one of them as "DOD"). Nor did Cooper ever claim "he flew directly over Area 51" -- the camera really "had a range of view of about 1500 miles". "As for the photos you say you were shown," Miller continued, "itís possible that some of them have been de-classified in the years since the mission, but the Colonel was unequivocally told that ALL of the film was placed under classification shortly after Gemini-5ís return to the carrier and President Johnson subsequently acknowledged directly to the Colonel that he had personally given the order." But the testimony of several experts much closer than Cooper to the actual photographic process stands in shocking -- and dismaying -- contrast to the version in the book. So do the cold equations of orbital motion, and the precise calculus of optics. And so, too, did the photographs and catalogs I held in my hands at the NASA photo lab.

The hints I've found were indicating it was a test of acoustic detection of rocket launchings, by existing and experimental DoD sensors around the world, using the launch as the 'signal'. As far as I can tell there was no hardware or any activity by the crew -- it was entirely passive.

Interesting but confusing. If that is what it was, then NASA did not have to be involved at all. This would have simply involved acoustic sites around the world operated by other agencies. Yet I believe that first mention of Chapel Bell appeared in a NASA document.

When the Soviet N-1 rocket blew up in July 1969, there are indications that it was detected in near-real-time, a few weeks before it showed up in satellite photos. But it's unclear if it was detected acoustically, or seismically.

Just some thoughts on Jim's story from my own work with astrophotography (same equipment pointed the other way)...

Contarex would be the camera, and Questar is (still) the high-end Maksutov-class telescope used in a telephoto configuration. In that slightly extended configuration, its focal length would be about 1300mm, close enough to Cooper's memory. The focal ratio of the telescope is about f/16, so by the "sunny 16" rule of photography (at f/16 in full sunlight, the exposure should be equivalent to the film's ISO value), the film speed was somewhere in the ASA 32-50 range. For B&W emulsions of the time, the fine-grained candidate would have been Panatomic-X (ASA 50, with a slight development push). Plus-X (ASA 125) was another candidate, but with the fixed f/16 focal ratio, it would have used 1/100 second for the exposure. Had they used Tri-X (ASA 400), the shutter speed could have been MUCH more hand-holdable, but at some loss of resolution. The grain of Tri-X in the day could best be described as "salt and pepper," although latest formulations are much improved.

The other applicable rule of photography is that you should not hand-hold a telephoto lens at any speed corresponding to less than 100mm of focal length (ie, 1/100 for 100mm, 1/1000 for 1000mm). It IS possible to pan with the subject and get a sharp exposure (as Cassini did with its "skeet shoot" pictures of Enceladus), but at 1/50 of a second and 1300mm focal length, a goodly number of the pictures should have been blurry, by the odds of the game.

Panatomic-X is a thin film, so a standard 35mm cassette could easily hold 55 exposures worth of film. Plus-X and Tri-X have thicker bases and that length would have been very tight in the film transport and take-up, not to mention rewinding again. On the other hand, the chances of successfully sharp shots would have been much greater with Tri-X, where a sharp but grainy picture is better than a blurry picture any day.

The view through a camera-mounted telephoto, to use another photographic rule of thumb, is roughly equivalent to binoculars whose power is 1/50 of the focal length, or about like looking through 26-power field glasses. Spotting targets through the equivalent of a spotting scope with the added dimness of SLR focusing screens makes it remarkable that he caught any targets in focus and without motion blur. Cooper was The Right Stuff, so of course he got the job done.

By the way, B&W film is extremely easy to develop on ship, so that scenario is plausible. Kodachrome likely would have been sent to a certified lab, as it was not end-user processible.